Amounts less than 0.1 m3 or 100 kg are considered too small to have
caused serious contamination because it is assumed that such quantities
can only contaminate limited amounts of soil or groundwater and that
this contamination, because of dilution, will never reach concentrations
that are high enough to cause health impacts. The amount has been
chosen arbitrarily and more on an empirical than a scientific basis.

DEGRADATION RATE

The rate of degradation of pesticides is often expressed as half-life
(DT50), expressed in years, months or days. Every pesticide has its
own half-life value. After this period only half of the original amount
of pesticide is left, the other half having been degraded away.
Degradation occurs because of biological organisms (bacteria, fungi)
and by physicochemical interactions.

The rate of degradation of various pesticides is influenced by external
factors such as temperature, light and soil acidity. As a rule, degradation
of a compound is considered complete after a period of five times the
half-life of that compound. Consequently, a pesticides spill will not
lead to soil contamination if the age of the spill exceeds five times the
half-life of the pesticides.

A DT50 value of one-half year for soil will be used as the criterion for
assessing whether or not a pesticide is relevant in causing
contamination. Only those pesticides with a DT50 value of one-half
year or less will be considered relevant. This value is also used in
Appendix 3.

For a pesticide with a half-life of a six months or less, the risk of soil
becoming contaminated only exists for a period of 2.5 years after the
release of the pesticide. After this period, the pesticide will have been
degraded. In this way, a differentiation is made between pesticide
spills that are more urgent because of the low degradation of the
pesticide, and those that are less urgent because the pesticides
concerned are subject to fast degradation.